1. Field of the Invention
The present invention pertains generally to shearing devices and more particularly to shearing devices for shearing printed circuit boards.
2. Discussion of the Background of the Invention
Shearing devices can be used in three separate applications in printed circuit board assembly lines including; (1) cutting panels from large printed circuit board sheet material, (2) trimming copper and epoxy flash on multilayered panels, and (3) depaneling bare boards and printed circuit boards.
In prior art processes, large sheets of printed circuit board material, which are normally on the order of 42".times.48" are sheared into smaller panels of approximately 18".times.24". Of course, these dimensions may vary. Each of the panels contains a multiplicity of individual printed circuit boards of various shapes and sizes. The conductive paths of each of the printed circuit boards are etched on the boards in the larger panels. Each panel can accommodate various numbers of boards depending upon the size and the shape of the individual printed circuit boards. A routing device is then used to substantially separate the individual circuit boards from the larger panel. Tab portions are left between the individual circuit boards in the panel so that the individual boards can be maintained in a single structure for loading electrical components, soldering in a wave-soldering machine and testing the printed circuit boards after soldering. Individual boards can then be depaneled by manually breaking the tabs. Depaneling can occur either before or after loading of the electrical components and soldering.
The above described method of routing is the most common method of shaping individual printed circuit boards (otherwise known as profiling) and separating the individual printed circuit boards from each other and from scrap portions (otherwise known as depaneling). Routing techniques allow considerable flexibility in shaping the printed circuit boards and various configurations including arcs and curves which are not possible with other types of devices such as shears. However, routing is an extremely slow process which is noisy and produces a large amount of debris during cutting. Additionally, since the routing blade has a predetermined minimum diameter, a predetermined amount of waste is necessarily built into the layout of printed circuit boards on the panels to accommodate the router blade. Consequently, as the board sizes become smaller, the amount of waste increases because of the inability of routers to provide zero spacing between the boards. Moreover, loaded boards cannot be cut with a router because of vibration and stress produced by the routing process which causes solder joints to break. Also, the physical design of the router does not permit sufficient clearance to prevent interference between the router and components mounted on the boards. Also, routers are large, requiring additional floor space, and are expensive to acquire and maintain. Additionally, routers require a skilled operator and consume a large amount of energy to operate. Also, the routing blades must be replaced frequently because they become dull very easily when cutting the printed circuit board material.
An additional problem resulting from the use of routers to separate individual printed circuit boards from larger panels is the time required to break the tabs comprising the supporting structure. Manual separation of the boards by breaking the tabs comprises a labor intensive process. Automated methods of breaking the tabs to separate the boards do not currently exist. Even if such automated methods did exist, this would require another step in the production line which would necessitate additional capital expenditures, complicate the production line and impede the flow of product.
As indicated above, the individual printed circuit boards are loaded with electrical components after the boards have been routed and the tabs formed. The structure is then placed in a wave soldering machine to solder the components to the printed circuit boards. The tab portions must have sufficient rigidity to support the structure with the components loaded thereon to ensure proper electrical connections are made in the wave soldering machine. At the same time, the tab portion must be sufficiently small to allow easy separation of the individual circuit boards from the larger panel without causing any breakage of the board or excess material.
Small board size produces a practical limit to automatically mass producing boards on a panel. As the board dimensions become smaller the linear routing requirements become more extensive, exacerbating the problems of routing set forth above, as well as causing other problems along the assembly line such as premature separation and wave solder overflow. This virtually prohibits the production of small boards in a fully automated process. Hence, manual fabrication of smaller boards is necessitated in an otherwise completely automated assembly line.
These problems have been overcome to some extent through the use of water jet cutters and laser cutting devices. Laser cutting devices, however, are only useful in cutting the fiberglass sheets prior to assembly in laminated sheets. Hence, laser cutters do not provide the capability of cutting loaded boards from a panel. Although water jet cutters are able to cut loaded printed circuit boards and are able to provide flexibility to the shape of the board to be cut from the panel, as well as reducing scrap in the cutting process, they are expensive to install and maintain. They are also dangerous to operate and require a large amount of floor space.
These problems have been overcome to some extent by the use of a shearing device which is capable of cutting the boards in a clean, quiet and rapid fashion with a device that does not require frequent blade replacement. Shearing devices also do not require preparation of the panels prior to depaneling as do routing devices. Shearing devices, in special circumstances, reduce the separation spacing of the individual printed circuit boards within the panel because of their ability to cut the boards at close intervals. However, shearing devices have generally not been employed in the cutting of printed circuit boards from panels because of the poor edge quality produced by the cut and safety considerations regarding shearing devices.
A patentability search was performed on the use of shears in manufacturing. The search produced the following art:
______________________________________ Patent No. Inventor Date of Issue ______________________________________ 2,626,664 Regele Jan. 27, 1953 2,963,627 Buchsbaum Dec. 6, 1960 3,064,512 Zurlo Nov. 20, 1962 3,195,387 Telfer July 20, 1965 3,771,401 Jasinski Nov. 13, 1973 3,834,213 Henzler et al. Sept. 10, 1974 4,070,940 McDaniel et al. Jan. 31, 1978 4,309,696 Nagai et al. Jan. 5, 1982 4,312,618 Greene Jan. 26, 1982 4,466,766 Geren, Aug. 21, 1984 deceased et al. Great Britain Pull Nov. 12, 1980 1,579,020 Germany Visomat-Gerate May 30, 1973 2,158,182 ______________________________________
McDaniel et al. discloses a shearing machine with a protective light curtain. Infrared light source 12 produces an infrared light beam which is reflected from corner reflector 46, across the front portion of the shearing machine and reflected back by a second corner reflector 51 to sensor unit 41. This produces a protective light curtain across the front of the machine such that insertion of any object as large as a finger will cause a reduction in voltage of the sensor unit 41, as illustrated in FIG. 3, to stop the cutting operation by opening switch 37. Column 4, lines 46+, disclose the operation of the protective light curtain. FIG. 2 also discloses a lower blade 28 which has a rectangular configuration which is secured to the inner edge of table 17 to interact with a beveled upper moveable blade 24.
The British Patent No. 1,579,020, the German Patent No. 2,158,182, Nagai et al. U.S. Pat. No. 4,309,696 and Buchsbaum Pat. No. 2,963,627, all relate to similar light ray safety devices.
The British patent relates to a device having multiple sensors and transmitters, i.e., sixteen light emitting diodes and receivers which produce an infrared light curtain. A sequencing of the sensors is used to obviate alignment problems.
The German patent discloses a device in which light is emitted from a common source and distributed by fiber optics or curved mirrors to produce a light safety curtain which is reflected back to received units. For example, FIG. 2 illustrates a light source 11 which projects a light beam which is focused by lens 12 and projected through a beam splitter 13 to a rotating square mirror 14 which distributes the beam of light progressively along curved mirror 15. A corner reflector 16 reflects the light back to curved mirror 15 and onto rotating reflector 14 to beam splitter 13 which reflects the return beam into lens 17 which focuses the return beam onto a single detector 18. Other methods are shown in FIGS. 1 and 3 using multiple light sources and multiple detector units.
Nagai et al. discloses a test circuit for detecting trouble in an optical security device using sequencing techniques.
Buchsbaum discloses an electronic guard device which uses an RF electromagnetic field rather than an optical frequency electromagentic field. As illustrated in FIGS. 5 and 6 and columns 7 and 8, a wiper arm 110 provides an electrical voltage which varies in accordance with the position of shaft 22. Potentiometer 111 produces a voltage which varies with the position of shaft 22. Transmitter circuit 29 produces an electrical field 103 which is detected by receiver and detector 101. As shaft 22 is inserted into the RF field 103, the voltage level is decreased at receiver and detector 101. The voltage produced by control generator 105 and the voltage produced by receiver detector 101 is compared in DC amplifier 104 to determine if these voltages match. If any other object is inserted within the RF field, voltages will not match and indicator relay 91 will activate to disengage shaft 22.
Henzler et al., Geren and Greene, all disclose specialized automated devices for handling work pieces. Henzler et al. discloses a work piece translation mechanism for transferring work pieces through a stamping die.
Jasinski discloses a sheet metal shearing apparatus which utilizes a moving upper blade and a stationary lower blade. The upper blade is driven in a vertical direction by a piston rod 74 in a fluid operated cylinder 76 to produce a centralized force on blade 60. As illustrated in FIG. 4, blade 60 is held by side supports which appear to be made of bearing type material.
Regele, Zurlo, and Telfer all disclose moveable upper blades which interact with a replaceable lower blade. Zurlo discloses a stationary and removable lower blade 2 which interacts with a pointed shear blade 1 having an extremely pointed shear edge 6. Regele discloses an upper cutter bar 14 which utilizes a progressive shearing action, as illustrated in FIG. 1, which is used to prevent a single fracture so as to produce a cut edge rather than a fractured edge, as disclosed in column 3, lines 43 through 50. As diclosed in column 2, lines 13 through 18, the lower blade 11 is removable. Additionally, hollow edge 48, as illustrated in FIG. 5, is used to provide a sharp bevel to provide a clean cut. Telfer discloses a removable lower blade 38, as illustrated in FIG. 3, which interacts with moveable upper blade 31. The lower blade 38 has a rectangular configuration.
Currently available printed circuit board shearing devices suffer from many disadvantages and limitations therein. For example, the design of currently available shearing devices utilize a non-centered actuation system which results in the production of lever arm forces on the cutting blade resulting in inaccurate cuts. This, "out-of-line" design results in a machine which is large and bulky, expensive to build, difficult and expensive to maintain, requires frequent replacement of bearings resulting in inconvenient and expensive "down time" situations, and is incapable of providing sufficient rigidity to produce consistent, accurate cuts for extended periods.
Moreover, current designs of shearing devices for printed circuit boards utilize rear tooling, comprising tooling pins that are actuated to engage tooling holes in the printed circuit board which are normally located on scrap portions of the printed circuit board panels. The lower blade configuration confines the current shearing devices to rear tooling, i.e., tooling which is located behind the cutting blade, because of the design of the lower cutting blade of these conventional shearing devices. Conventional shearing devices use a horizontally disposed lower cutting blade which is attached to a lower blade holder in a horizontal orientation. This configuration does not provide room for tooling in front of the blade which is capable of engaging useable or interior portions of the printed circuit board panels. Panels cannot, therefore, be designed with zero spacing between the printed circuit boards. Rather, the panels must be designed with tooling holes in the border portions for locating the panels during the shearing process. Throughout the assembly line process locating tooling engages existing tooling holes on the interior portion of the panel. Consequently, conventionally designed shearing devices require specially drilled panels having tooling locating holes placed in custom designed border portions. This requires specially designed panels that are not needed for any other portion of the assembly line process.
Additionally, conventional shearing devices such as the shearing device produced by CENCORP, 4575 North Eleventh Street, Boulder, Colo. 80306 has a lip portion on the lower blade which is inherently weak. The lip portion is vertically displaced from the remainder of the horizontally oriented blade and provides some room for leads protruding beneath the panel in front of the blade near the cutting surface. The vertical displacement of the cutting portion of conventional lower blade devices provides minimal and normally inadequate clearance for component leads. Additional clearance cannot be provided because of the further weaknesses which would be caused by extending the cutting portion of the horizontally disposed blade in a vertical direction. Because of the inherent weaknesses of such a design, a minimal cross-sectional area of blade is required, which further restricts the spatial proximity which can be achieved between components mounted on the board and location of the cut because of the clearance needed for the leads of these components.
Currently available shearing devices also typically utilize archaic safety systems which include the use of a light curtain disposed in front of the cutting blade a considerable distance. Most shears are not designed for use with light curtains and these are noramlly added as a modification of the shear. Since the optical curtain is not aligned with the cutting blades, various errors can be made in detecting a potential obstruction since the optical detector is not actually detecting obstructions in the plane of movement of the cutting blade, but rather obstructions within a plane adjacent the plane of movement of the cutting blade. Additionally, such systems normally detect the integrated return of the optical curtain and make no attempt to determine the spatial location of the obstruction relative to the blade. This built-in "error factor" can also result in errors in detecting obstructions of the blade.
Consequently, conventional shearing devices have various disadvantages and limitations inherent in the design of these devices.